1. Field of the Invention
The subject invention relates to a rain sensor for detecting moisture on the windshield of a vehicle for turning on the windshield wipers, moisture taking the form of mist, water drops or film, frost, ice, snow, fog, etc.
2. Description of the Prior Art
Many imaging rain sensors detect the sharp edges of raindrops imaged onto a photodiode focal plane array, i.e., a camera. This type is described in U.S. Pat. No. 5,923,027 to Stam et al. Ambient lighting and auxiliary lighting for low light operation is generally non-critical in such edge detection sensors. Since primary image discrimination in the prior art is provided by the image contrast of raindrop edges, any light source that delivers suitable background/object contrast is generally acceptable. Of course, the selectivity and sensitivity of such edge detection methods is limited because of interfering light sources and the inability of the optics to limit sensitivity to the windshield surface. Despite attempts to defocus specular (interfering) sources, moving bright lights within the detector field of view tend to confuse the control system, causing unwanted (phantom) wiping.
The prior art which sometimes uses ancillary illumination needs only to flood the field of view with sufficient radiant energy to allow the imager to discern rain drop edges. That is, the specific geometric relationship between the radiant source, the windshield and the imager is not critical and ill defined.
The U.S. Pat. No. 5,313,072 to Vachss uses an imager array to sense the backscattered light from raindrops and does specify a geometric relationship between the light source, windshield and imager. However, as shown in the Vachss ""072 patent, the illumination source would completely overwhelm the imager with direct or reflected illuminant, thereby rendering the sensor blind to the relatively weak rain drop reflection signals.
In a typical automotive installation, the interior placement of the illuminator and geometry required by the prior art would result in approximately 14% of the illuminator energy reaching the imager (on axis). At the optimal near infrared wavelength of 880 nm, typical automotive windshields only exhibit approximately 10% transmission. For a two pass, send and return path, the maximum illuminant that could be direct to the imager from a perfectly reflective object on the glass, would be about 1%. For small raindrops with a nominal diameter of 1 mm in a visual field of 104 mm2, the expected return (per drop) is well under 106 (one-millionth) the output of the IR emitting illuminator. Resolving signals from raindrops that are five orders of magnitude less than the interfering (on axis) signal is very difficult, and is the main reason that the on-axis illumination of prior art rain sensors has not been practical.
The present invention avoids the problems of such direct reflection of the illuminator by the inside glass surface of the windshield. Instead of intercepting the xe2x80x98on axisxe2x80x99 illuminant as in the prior art, the invention uses one or more illuminator arrays or strips to irradiate the windshield in a topology that shields the detector (camera) from direct/reflected on axis radiation of the illuminator.
The invention provides a method and assembly for sensing moisture on the exterior surface of a sheet of glass by emitting light rays from an illuminator on an illuminator axis intersecting the glass at an illuminator angle of incidence to reflect the light rays on a reflection axis at an angle of reflection to the glass, and capturing on an imaging axis the reflected light rays including reflections of moisture on the exterior surface of the glass. The invention is characterized by isolating the reflection of moisture from the direct reflection of the illuminator to prevent the capture of the direct reflection of the illuminator.